The lentivirus equine infectious anemia virus (EIAV), displays a highly restricted cell type preference both in vitro and in vivo. Additionally, like other members of the lentivirus family, EIAV is subject to antigenic variation as evidenced by changes that occure in envelope glycoproteins during the course of infection. To further understand the restrictive host range and correlate proviral changes that occur during pathogenesis, we have molecularly cloned and sequenced an EIAV provirus. Comparison of gag and pol genes of EIAV with human immunodeficiency virus (HIV) and visna clearly establishes that EIAV is genetically related and equally divergent from these two distinct lentiviruses. We have examined the long terminal repeats (LTRs) of EIAV with respect to their ability to function as transcriptional promoters in various cellular environments. Nucleotide sequence analyses of the LTRs derived from two unique proviral clones revealed consensus transcription and processing signals. One of the proviruses possessed a duplication of a 16 base pair (bp) sequence in the CCAAT box region which was absent in the other provirus. To assess their functional activity, each LTR was coupled to the bacterial chloramphenicol acetyl transferase (CAT) gene and transfected onto various cell lines, including matched cultures of EIAV-infected and -uninfected cells. The levels of CAT activity directed by the EIAV LTRs were between 250 and 900 times greater in EIAV-infected cells compared to uninfected counterparts. Thus, EIAV expression appears to be subject to a virus-induced transactivation analogous to that recently shown to amplify expression of certain other lentiviruses including HIV. To broaden our understanding of lentivirus viral gene regulators and to acquire additional reagents to examine and detail the mechanism of lentiviral transactivation, we have begun to molecularly clone the simian lentivirus macaque immunodeficient virus (MnIV).